1. Field of the Invention
The invention is in the field of commercial chemical purification and separation; more particularly, in the field of commercial processes for purifying vinyl acetate monomer which has become impure or contaminated.
2. Description of the Prior Art
Vinyl acetate monomer is commercially prepared from acetylene and acetic acid. The vapor-phase reaction between acetylene and acetic acid in the presence of a zinc acetate catalyst yields vinyl acetate.
Acetylene is specially purified to remove hydrogen sulfide and phosphorus compounds. It is then mixed in slight excess with vaporized acetic acid and fed to a multitubular fixed-bed reactor containing a catalyst of zinc acetate deposited on activated carbon (10 percent Zn). Reaction is exothermic so the reactor is cooled by circulating oil around the tubes. Reactor temperature is maintained at 350.degree. to 400.degree. F. (177.degree. to 204.degree. C.). The reactor effluent is condensed and fed to a light ends column, where acetylene, methyl acetylene, propadiene, and other light ends are removed from the top of the column. The acetylene must be repurified before it may be recycled.
Vinyl acetate is distilled overhead in a vinyl acetate column. Recycle acetic acid is separated from heavy ends in a recovery column.
Vinyl acetate monomer, which has the chemical formula CH.sub.3 COOCH.dbd.CH.sub.2, is supplied commercially in grades which differ in the amount of chemical inhibitor they contain but otherwise have identical specifications. Typical manufacturers' specifications are:
______________________________________ Vinyl acetate, % 99.8, min Boiling point, .degree.C. 72.3-73.0 Acidity as acetic acid, % by wt. 0.005, max Carbonyls as acetaldehyde, % by wt. 0.013, max Water, % by wt. 0.04, max Color APHA system 0-5 Suspended matter None ______________________________________
Despite the use of chemical inhibitors, a significant commercial problem exists in that bulk quantities of vinyl acetate monomer in storage or in transportation tanks frequently build up acetic acid and coloration agents during hot weather due to the heat and the presence of oxygen or metal ions which may cause decomposition of the monomer to form acetic acid and the coloration agents. Another commercial problem which may occur is that vinyl acetate monomer will pick up sodium chloride during shipment or storage.
At the present time, manufacturers of vinyl acetate monomer have specifications of 0.005% by weight acetic acid (max.) and 0.04% by weight water (max.). Furthermore, the monomer should be water white, i.e., have no color, and be free of sodium chloride.
When a bulk quantity of vinyl acetate monomer fails to satisfy the standard specifications because of excess acetic acid, water, sodium chloride, and/or coloration, the monomer either will be sold at a significant discount or it will be re-distilled which is an energy-intensive and, therefore, expensive process.